In today's radio communications networks a number of different technologies are used, such as Long Term Evolution (LTE), LTE-Advanced, Wideband Code Division Multiple Access (WCDMA), Global System for Mobile communications/Enhanced Data rate for GSM Evolution (GSM/EDGE), Worldwide Interoperability for Microwave Access (WiMax), or Ultra Mobile Broadband (UMB), just to mention a few possible technologies for radio communication. A radio communications network comprises radio base stations providing radio coverage over at least one respective geographical area forming a cell. The cell definition may also incorporate frequency bands used for transmissions, which means that two different cells may cover the same geographical area but using different frequency bands. User equipments (UE) are served in the cells by the respective radio base station and are communicating with respective radio base station. The user equipments transmit data over an air or radio interface to the radio base stations in uplink (UL) transmissions and the radio base stations transmit data over an air or radio interface to the user equipments in downlink (DL) transmissions.
Long Term Evolution (LTE) is a project within the 3rd Generation Partnership Project (3GPP) to evolve the WCDMA standard towards the fourth generation (4G) of mobile telecommunication networks. In comparisons with third generation (3G) WCDMA, LTE provides increased capacity, much higher data peak rates and significantly improved latency numbers. For example, the LTE specifications support downlink data peak rates up to 300 Mbps, uplink data peak rates of up to 75 Mbit/s and radio access network round-trip times of less than 10 ms. In addition, LTE supports scalable carrier bandwidths from 20 MHz down to 1.4 MHz and supports both Frequency Division Duplex (FDD) and Time Division Duplex (TDD) operation.
LTE technology is a mobile broadband wireless communication technology in which transmissions are sent using orthogonal frequency division multiplexing (OFDM), wherein the transmissions are sent from base stations, also referred to herein as network nodes or eNBs, to mobile stations, also referred to herein as user equipments or UEs. The transmission OFDM splits the signal into multiple parallel sub-carriers in frequency.
A basic unit of transmission in LTE is a Resource Block (RB) which in its most common configuration comprises 12 subcarriers and 7 OFDM symbols in one time slot. A unit of one subcarrier and 1 OFDM symbol is referred to as a resource element (RE), as shown in FIG. 1. Thus, an RB comprises 84 REs.
Accordingly, a basic LTE downlink physical resource may thus be seen as a time-frequency grid as illustrated in FIG. 1, where each Resource Element (RE) corresponds to one OFDM subcarrier during one OFDM symbol interval. A symbol interval comprises a cyclic prefix (cp), which cp is a prefixing of a symbol with a repetition of the end of the symbol to act as a guard band between symbols and/or facilitate frequency domain processing. Frequencies for subcarriers having a subcarrier spacing Δf are defined along an z-axis and symbols are defined along an x-axis.
In the time domain, LTE downlink transmissions are organized into radio frames of 10 ms, each radio frame comprising ten equally-sized sub-frames, #0-#9, each with a Tsub-frame=1 ms of length in time as shown in FIG. 2. Furthermore, the resource allocation in LTE is typically described in terms of resource blocks, where a resource block corresponds to one slot of 0.5 ms in the time domain and 12 subcarriers in the frequency domain. Resource blocks are numbered in the frequency domain, starting with resource block 0 from one end of the system bandwidth.
An LTE radio sub-frame is composed of multiple RBs in frequency with the number of RBs determining the bandwidth of the system and two slots in time, as shown in FIG. 3. Furthermore, the two RBs in a sub-frame that are adjacent in time may be denoted as an RB pair.
Downlink transmissions are dynamically scheduled in the current downlink subframe. This means that, in each subframe, the network node transmits control information about to which UEs data is transmitted, and upon which resource blocks the data is transmitted. This control signalling is typically transmitted in the first 1, 2, 3 or 4 OFDM symbols in each subframe denoted the control region. In FIG. 3, for example, a downlink system with 1 out of 3 possible OFDM symbols as control signalling is illustrated.
The dynamic scheduling information is communicated to the UEs via a Physical Downlink Control CHannel (PDCCH) transmitted in the control region. After successful decoding of a PDCCH, the UE performs reception of the Physical Downlink Shared CHannel (PDSCH) or transmission of the Physical Uplink Shared CHannel (PUSCH) according to a pre-determined timing specified in the LTE specification.
Furthermore, LTE uses Hybrid-ARQ (HARQ). That is, after receiving DL data in a subframe, the UE attempts to decode it and reports to the network node an Acknowledgement (ACK) or a Non-Acknowledgement (NACK) if the decoding was successful or not successful. This is performed via the Physical Uplink Control CHannel (PUCCH). In case of an unsuccessful decoding attempt, the network node may retransmit the erroneous data.
Similarly, the network node may indicate to the UE an Acknowledgement (ACK) or a Non-Acknowledgement (NACK) if the decoding of the PUSCH was successful or not successful via the Physical Hybrid ARQ Indicator CHannel (PHICH).
The DL Layer-1/Layer 2 (L1/L2) control signalling transmitted in the control region comprises the following different physical-channel types:                The Physical Control Format Indicator CHannel (PCFICH). This informs the UE about the size of the control region, e.g. one, two, or three OFDM symbols for system bandwidths larger than 10 RBs and two, three or four OFDM symbols for system bandwidths equal to 10 RBs or smaller. There is one and only one PCFICH on each component carrier or, equivalently, in each cell.        The Physical Downlink Control CHannel (PDCCH). This is used to signal DL scheduling assignments and UL scheduling grants. Each PDCCH typically carries signalling for a single UE, but can also be used to address a group of UEs. Multiple PDCCHs can exist in each cell.        The Physical Hybrid-ARQ Indicator CHannel (PHICH). This is used to signal hybrid-ARQ acknowledgements in response to UL-SCH transmissions. Multiple PHICHs can exist in each cell.        
These physical channels are organized in units of Resource Element Group (REG), which comprises four closely spaced resource elements. The PCFICH occupies four REGs and a PHICH group occupies three REGs. An example of control channels in an LTE control region, assuming a system bandwidth of 8 RBs, is shown in FIG. 4.
Physical Downlink Control CHannel (PDCCH)
The PDCCH is used to carry Downlink Control Information (DCI), such as, e.g. scheduling decisions and power-control commands. More specifically, the DCI comprises:                Downlink scheduling assignments. These may comprise PDSCH resource indication, transport format, hybrid-ARQ information, and control information related to spatial multiplexing (if applicable). A downlink scheduling assignment also comprises a command for power control of the PUCCH used for transmission of hybrid-ARQ acknowledgements in response to downlink scheduling assignments.        Uplink scheduling grants. These comprise PUSCH resource indication, transport format, and hybrid-ARQ-related information. An uplink scheduling grant also comprises a command for power control of the PUSCH.        Power-control commands for a set of UEs, which may serve as a complement to the commands comprised in the scheduling assignments/grants.        
As multiple UEs may be scheduled simultaneously, on both DL and UL, there must be a possibility to transmit multiple scheduling messages within each subframe. Each scheduling message is transmitted on a separate PDCCH, and consequently there are typically multiple simultaneous PDCCH transmissions within each cell. To accommodate multiple UEs, LTE defines so-called search spaces. The search spaces describe the set of CCEs which the UE is supposed to monitor for scheduling assignments/grants relating to a certain component carrier. A UE has multiple search spaces, namely, UE-specific search spaces and the common search space.
Fast link adaptation to a fading channel condition is used in radio communication network to enhance system throughput capacity, as well as, user experience and quality of services. An important factor in the working of fast link adaptation is the timely update of channel conditions that is fed back from the receiver to the transmitter. The feedback may take on several related forms, such as, e.g. a signal to noise ratio (SNR), a signal to interference and noise ratio (SINR), a received signal level (e.g. power or strength), supportable data rates, supportable combination of modulation and coding rates, supportable throughputs, etc. The information may also pertain to entire frequency bands, as in W-CDMA systems, or specific portions of it as made possible by systems based OFDM such as the LTE system. These feedback messages may generally be referred to as a Channel Quality Indicator (CQI).
In DL data operations in LTE, the CQI messages are fed back from the UE to the network node to assist the transmitter in the network node in the decision of radio resource allocation. The feedback information may, for example, be used to determine transmission scheduling among multiple receivers; to select suitable transmission schemes, such as, e.g. the number of transmit antennas to activate; to allocate appropriate amount of bandwidth; and to form supportable modulation and coding rate for the intended receiver in the UE.
In UL data operations in LTE, the network node may estimate the channel quality from the Demodulation Reference Symbols (DRS) or Sounding Reference Symbols (SRS) transmitted by the UEs.
The range of a CQI message in LTE is shown in the CQI message table of FIG. 5. This table is the table 7.2.3-1 present in the standard specification 3GPP TS 36.213 “Physical Layer Procedures”. This CQI message table has been specifically designed to support Modulation and Coding Scheme (MCS) adaptation over wide-band wireless communication channels. The transition points from a lower-order modulation to a higher-order modulation have been verified with extensive link performance evaluation. These specific transition points between different modulations thus provide a guideline for well-adjusted system operation.
Based on the CQI message from a UE, a network node may choose the best MCS to transmit data on the PDSCH. The MCS information is conveyed to the selected UE in a 5-bit “modulation and coding scheme” field (IMCS) of the DCI, as shown in the MCS table of FIG. 6. The MCS field IMCS signals to the UE both the modulation Qm and the transport block size (TBS) index ITBS. In conjunction with the total number of allocated RBs, the TBS index ITBS further determines the exact transport block size used in the PDSCH transmission. The last three MCS entries are for HARQ retransmissions and, hence, the TBS remains the same as the original transmission.
The specific TBSs for different number of allocated radio blocks are defined and listed for the single layer transmission case in the TBS table 7.1.7.2.1-1, i.e. a large 27×110 table, in the standard specification 3GPP TS 36.213 “Physical Layer Procedures”. However, these TBSs are designed to achieve spectral efficiencies matching the CQI messages. More specifically, the TBSs are selected to achieve the spectral efficiencies shown in the table of FIG. 7.
Note that the CQI message table in FIG. 5 and, consequently, the MCS table of FIG. 6, are both designed based on the assumption that 11 OFDM symbols are available for PDSCH transmission. This means that when the actual number of available OFDM symbols for PDSCH is different than 11, the spectral efficiency of the transmission will deviate from the spectral efficiencies shown in the table of FIG. 7.
Enhanced Control Channel (eCCH)
Transmission of a Physical Downlink Shared CHannel (PDSCH) to UEs may use REs in RB pairs that are not used for control messages or RS. Further, the PDSCH may either be transmitted using the UE-specific reference symbols or the CRS as a demodulation reference, depending on the transmission mode. The use of UE-specific RS allows a multi-antenna network node to optimize the transmission using pre-coding of both data and reference signals being transmitted from the multiple antennas so that the received signal energy increases at the UE. Consequently, the channel estimation performance is improved and the data rate of the transmission could be increased.
In LTE Release 10, a Relay Physical Downlink Control CHannel is also defined and denoted R-PDCCH. The R-PDCCH is used for transmitting control information from network node to Relay Nodes (RN). The R-PDCCH is placed in the data region, hence, similar to a PDSCH transmission. The transmission of the R-PDCCH may either be configured to use CRS to provide wide cell coverage, or RN specific reference signals to improve the link performance towards a particular RN by precoding, similar to the PDSCH with UE-specific RS. The UE-specific RS is in the latter case used also for the R-PDCCH transmission. The R-PDCCH occupies a number of configured RB pairs in the system bandwidth and is thus frequency multiplexed with the PDSCH transmissions in the remaining RB pairs, as shown in FIG. 8.
FIG. 8 shows a downlink sub-frame showing 10 RB pairs and transmission of 3 R-PDCCH, that is, red, green or blue, of size 1 RB pair each. The R-PDCCH does not start at OFDM symbol zero to allow for a PDCCH to be transmitted in the first one to four symbols. The remaining RB pairs may be used for PDSCH transmissions.
In LTE Release 11 discussions, attention has turned to adopt the same principle of UE-specific transmission as for the PDSCH and the R-PDCCH for enhanced control channels, that is, including PDCCH, PHICH, PBCH, and Physical Configuration Indication CHannels (PCFICH). This may be done by allowing the transmission of generic control messages to a UE using such transmissions to be based on UE-specific reference signals. This means that precoding gains may be achieved also for the control channels. Another benefit is that different RB pairs may be allocated to different cells or different transmission points within a cell. Thereby, inter-cell interference coordination between control channels may be achieved. This frequency coordination is not possible with the PDCCH, since the PDCCH spans the whole bandwidth.
FIG. 9 shows an enhanced PDCCH (ePDCCH) which, similar to the CCE in the PDCCH, is divided into multiple groups (eREG) and mapped to one of the enhanced control regions. However, it should be noted that the relation between ePDCCH, eREGs and REs is not yet determined in the 3GPP standard. One option could be that the relation between ePDCCH and eREGs/REs are to be similar to that as for PDCCH, i.e. that one ePDCCH is divided into one or multiple eCCE(s) corresponding to 36 REs, which in turn is divided into 9 eREGs each comprising 4 REs. Another option may be to have one eCCE corresponding to up to 36 REs, and wherein each eREG corresponds to 18 REs. According to yet another option, it may be decided that the eCCE should correspond to even more than 36 REs, such as, e.g. 72 or 74.
That is, FIG. 9 shows a downlink sub-frame showing a CCE belonging to an ePDCCH that is mapped to one of the enhanced control regions, to achieve localized transmission.
Note that, in FIG. 9, the enhanced control region does not start at OFDM symbol zero, to accommodate simultaneous transmission of a PDCCH in the sub-frame. However, as was mentioned above, there may be carrier types in future LTE releases that do not have a PDCCH, in which case the enhanced control region could start from OFDM symbol zero within the sub-frame.
Time Division Duplex (TDD)
Transmission and reception from a UE may be multiplexed in the frequency domain, in the time domain or in a combination of the two domains, such as, e.g. the half-duplex FDD. FIG. 10 shows an illustration of Frequency Division Duplex (FDD) and Time Division Duplex (TDD).
Frequency Division Duplex (FDD) implies that DL and UL transmissions take place in different, sufficiently separated, frequency bands, while Time Division Duplex (TDD) implies that DL and UL transmissions take place in different, non-overlapping time slots. Thus, TDD may operate in an unpaired spectrum, whereas FDD requires a paired spectrum.
Typically, the structure of the transmitted signal is organized in the form of a frame structure. For example, LTE uses ten equally-sized subframes of length 1 ms per radio frame as illustrated in FIGS. 2 and 11.
As shown in the upper part of FIG. 11, in case of FDD operation, there are two carrier frequencies; one carrier frequency for UL transmission (FUL) and one carrier frequency for DL transmission (FDL). At least with respect to the UE, FDD may either be full duplex or half duplex. In the full duplex case, a UE may transmit and receive simultaneously, while in half-duplex operation, the UE cannot transmit and receive simultaneously. However, it should be noted that the network node is capable of simultaneous reception or transmission, e.g. receiving from one UE while simultaneously transmitting to another UE. In LTE, a half-duplex UE is monitoring or receiving in the DL except when explicitly being instructed to transmit in a certain subframe.
As shown in the lower part of FIG. 11, in case of TDD operation, there is only a single carrier frequency, and UL and DL transmissions are always separated in time and also on a cell basis. As the same carrier frequency is used for UL and DL transmission, both the network node and the UEs need to switch from transmission to reception and vice versa. An important aspect of any TDD system is to provide the possibility for a sufficiently large guard time, where neither DL nor UL transmissions occur. This is required in order to avoid interference between UL and DL transmissions. For LTE, this guard time is provided by special subframes, e.g. subframe #1 and, in some cases, subframe #6. These are then split into three parts: a downlink part (DwPTS), a guard period (GP), and an uplink part (UpPTS). The remaining subframes are either allocated to UL or DL transmission.
TDD allows for different asymmetries in terms of the amount of resources allocated for UL and DL transmission, respectively, by means of different UL and DL configurations. As shown in FIG. 12, there are seven different configurations in LTE. It should be noted that a DL subframe may mean either a DL subframe or the special subframe.
The LTE system has been designed to support a wide range of operation modes comprising the FDD and the TDD modes. Each of these modes may also operate with normal cyclic prefix (CP) lengths for typical cell sizes or with extended CP lengths for large cell sizes. To facilitate DL to UL switching, some special TDD subframes are configured to transmit user data in the DwPTS with shortened duration.
Furthermore, in the LTE system, available resources may be dynamically appropriated between control information and user data information. For example, the radio resource in a normal subframe is organized into 14 OFDM symbols. The LTE system may dynamically use {0, 1, 2, 3} OFDM symbols or {0, 2, 3, 4} OFDM symbols in case of very small system bandwidths to transmit control information. As a result, the actual number of OFDM symbols available for data transmission is 14, 13, 12, 11 or 10.
A summary of the number of available OFDM symbols for PDSCH transmission in different operation modes is given in the table of FIG. 13.
As previously mentioned, the CQI message table in FIG. 5 and, consequently, the MCS table of FIG. 6, are both designed based on the assumption that 11 OFDM symbols are available for PDSCH transmission. As shown in the table in FIG. 13, there are many cases where the actual resource available for transmission does not match this assumption. Thus, this assumption may lead to mismatch problems when the actual number of OFDM symbols available for PDSCH deviates from the assumed 11 OFDM symbols, which consequently will reduce data throughput.